Discrete Photopatternable π-Conjugated Oligomers for Electrochromic

Jul 2, 2008 - Three discrete oligomeric systems including an all-thiophene (T6) system, a thiophene/phenylene (TPTTPT) system, and a ...
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Discrete Photopatternable π-Conjugated Oligomers for Electrochromic Devices Christian B. Nielsen, Alex Angerhofer, Khalil A. Abboud, and John R. Reynolds* The George and Josephine Butler Polymer Research Laboratory, Department of Chemistry, Center for Macromolecular Science and Engineering, UniVersity of Florida, GainesVille, Florida 32611 Received January 4, 2008; E-mail: [email protected]

Abstract: Three discrete oligomeric systems including an all-thiophene (T6) system, a thiophene/phenylene (TPTTPT) system, and a thiophene/EDOT/phenylene (TPEEPT) system have been constructed and characterized with emphasis on structural, optical, electrochemical, and spectroelectrochemical properties. For all three chromophores, the radical cation, the dication, and the π-dimer have been identified and characterized. EPR spectroscopy reveals that the radical cations of TPTTPT and TPEEPT have g values of 2.008-2.012 and peak-to-peak widths in the range 4.2-5.3 G. Formation of the radical cation takes place at a lower potential for TPEEPT than for TPTTPT and T6, whereas subsequent oxidation to the dication occurs more easily for TPTTPT than for TPEEPT and T6. We ascribe this observation to more localized charges in the oxidized species of TPEEPT, which is supported by our finding that the radical cation of TPEEPT is less prone to undergo π-dimerization than the radical cations of TPTTPT and T6. All the oxidized species are sufficiently stable to allow for optical characterization, and the relative positions of all absorption bands are found to be in agreement with the electrochemical data. For further solid-state modifications of these materials, we have effectively modified the synthetic design and grafted terminal functionalities (e.s. acrylates) onto the discrete oligomers. Of these novel materials, TPEEPT proves to be the most promising anodically coloring material for electrochromics, and it undergoes reversible switching between two different colored states (bright yellow and clear blue) and one almost transparent and color neutral state. Contrast ratios, measured as ∆%T at λmax, are as high as 62.5%, and switching times are in the range 2-5 s for the coloration process, though significantly longer for the bleaching process. As a proof of concept, we have successfully constructed a simple photopatterned electrochromic device by exploiting the terminal acrylate functionalities of the oligomers in a UV-initiated cross-linking process. To the best of our knowledge, this is the first oligomer-based photopatterned electrochromic device reported in the literature.

Introduction

Ever since the discovery of electrical conductivity in doped polyacetylene, extensive research has been focused on new conjugated materials, their design, synthesis, and application in electronic devices.1 This class of materials primarily includes a vast number of conjugated polymers, but also more welldefined systems such as dendrimers and discrete oligomers.2–6 Considering the discrete π-conjugated oligomers, they not only serve as important model compounds for understanding fundamental properties of π-conjugated polymers, but also have been shown to possess attractive properties themselves for application

(1) Shirakawa, H.; Louis, E. J.; MacDiarmid, A. G.; Chiang, C. K.; Heeger, A. J. J. Chem. Soc., Chem. Commun. 1977, 1977, 578–580. (2) Lo, S. C.; Burn, P. L. Chem. ReV. 2007, 107, 1097–1116. (3) Mullen, K.; Wegner, G. Electronic Materials: The Oligomer Approach; Wiley-VCH: Weinheim, Germany, 1998. (4) Martin, R. E.; Diederich, F. Angew. Chem., Int. Ed. 1999, 38, 1350– 1377. (5) Fichou, D. J. Mater. Chem. 2000, 10, 571–588. (6) Skotheim, T. A.; Reynolds, J. R. Handbook of Conducting Polymers; Marcel Dekker: New York, 2007. 9734

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in electroactive devices such as organic field-effect transistors,7–12 organic light-emitting diodes,13–16 and in photovoltaic devices, either on their own or as part of a donor-acceptor system with fullerenes.17–20 The major advantage of discrete oligomers (7) Facchetti, A.; Deng, Y.; Wang, A. C.; Koide, Y.; Sirringhaus, H.; Marks, T. J.; Friend, R. H. Angew. Chem., Int. Ed. 2000, 39, 4547– 4551. (8) Yoon, M. H.; Facchetti, A.; Stern, C. E.; Marks, T. J. J. Am. Chem. Soc. 2006, 128, 5792–5801. (9) Murphy, A. R.; Frechet, J. M. J. Chem. ReV. 2007, 107, 1066–1096. (10) Horowitz, G.; Fichou, D.; Peng, X. Z.; Xu, Z. G.; Garnier, F. Solid State Commun. 1989, 72, 381–384. (11) Dodabalapur, A.; Torsi, L.; Katz, H. E. Science 1995, 268, 270–271. (12) Dimitrakopoulos, C. D.; Purushothaman, S.; Kymissis, J.; Callegari, A.; Shaw, J. M. Science 1999, 283, 822–824. (13) Geiger, F.; Stoldt, M.; Schweizer, H.; Bauerle, P.; Umbach, E. AdV. Mater. 1993, 5, 922–925. (14) Gigli, G.; Barbarella, G.; Favaretto, L.; Cacialli, F.; Cingolani, R. Appl. Phys. Lett. 1999, 75, 439–441. (15) Mitschke, U.; Bauerle, P. J. Mater. Chem. 2000, 10, 1471–1507. (16) Hung, L. S.; Chen, C. H. Mater. Sci. Eng., R 2002, 39, 143–222. (17) Jorgensen, M.; Krebs, F. C. J. Org. Chem. 2004, 69, 6688–6696. (18) Huisman, C. L.; Huijser, A.; Donker, H.; Schoonman, J.; Goossens, A. Macromolecules 2004, 37, 5557–5564. (19) Otsubo, T.; Aso, Y.; Takimiya, K. J. Mater. Chem. 2002, 12, 2565– 2575. 10.1021/ja7112273 CCC: $40.75  2008 American Chemical Society

Discrete Photopatternable π-Conjugated Oligomers

compared to their polymeric counterparts is monodispersity and hence a more direct and unambiguous correlation between structure and properties. From a synthetic point of view, the synthesis of discrete oligomers will often be more tedious and time-consuming than synthesis of conjugated polymers. However, structural complexity, such as incorporation of reactive end groups, can often be more easily achieved for well-defined oligomers than for polymers as a consequence of the different synthetic approaches. Here, we report on a new family of discrete conjugated oligomers and their electrochromic properties. Whereas conjugated polymers (e.g., polyanilines, polypyrroles, and polythiophenes), which have been used widely for electrochromic applications,21–26 generally have broad electronic absorption bands, discrete oligomers have more narrow and well-defined electronic absorption bands and are thus expected to undergo more clear and distinct color changes during the redox processes exploited for electrochromic behavior. This has been investigated nicely by Shirota and co-workers, who have reported on the electrochromic behavior of several polymeric systems containing small isolated chromophores.27–29 By attaching conjugated oligomers of varying length to vinyl and methacrylate polymers as pendant groups, they were able to create several electrochromes, which all underwent clear and reversible color changes (i.e., from blue to pale yellow, green to yellow, and orange to purple) upon oxidation. Similar systems have been described by other researchers including Barbarella and co-workers and Ferraris and co-workers.30,31 As opposed to attaching the isolated chromophores as a pendant group, they can also be incorporated into a polymer backbone using one of several approaches. As one example, McCulloch and co-workers have illustrated this well by synthesizing liquid crystalline organic semiconductors with photopolymerizable end groups;32,33 the isolated chromophores could thus be incorporated into a highly ordered crosslinked network by UV-curing subsequent to film processing. Inspired by several of the aforementioned contributions to recent advances in electrochromism and oligomer design and (20) Segura, J. L.; Martin, N.; Guldi, D. M. Chem. Soc. ReV. 2005, 34, 31–47. (21) Argun, A. A.; Aubert, P. H.; Thompson, B. C.; Schwendeman, I.; Gaupp, C. L.; Hwang, J.; Pinto, N. J.; Tanner, D. B.; MacDiarmid, A. G.; Reynolds, J. R. Chem. Mater. 2004, 16, 4401–4412. (22) Garnier, F.; Tourillon, G.; Gazard, M.; Dubois, J. C. J. Electroanal. Chem. 1983, 148, 299–303. (23) Hyodo, K. Electrochim. Acta 1994, 39, 265–272. (24) (a) Mortimer, R. J.; Dyer, A. L.; Reynolds, J. R. Displays 2006, 27, 2–18. (b) Dyer, A. L.; Reynolds, J. R. In Handbook of Conducting Polymers, 3rd ed.; Skotheim, T. A., Reynolds, J. R., Eds.; CRC Press: Boca Raton, FL, 2007. (25) Sapp, S. A.; Sotzing, G. A.; Reynolds, J. R. Chem. Mater. 1998, 10, 2101–2108. (26) Thompson, B. C.; Kim, Y. G.; McCarley, T. D.; Reynolds, J. R. J. Am. Chem. Soc. 2006, 128, 12714–12725. (27) Ohsedo, Y.; Imae, I.; Shirota, Y. J. Polym. Sci., Part B: Polym. Phys. 2003, 41, 2471–2484. (28) Shirota, Y. J. Mater. Chem. 2000, 10, 1–25. (29) Nawa, K.; Miyawaki, K.; Imae, I.; Noma, N.; Shirota, Y. J. Mater. Chem. 1993, 3, 113–114. (30) Melucci, M.; Barbarella, G.; Zambianchi, M.; Benzi, M.; Biscarini, F.; Cavallini, M.; Bongini, A.; Fabbroni, S.; Mazzeo, M.; Anni, M.; Gigli, G. Macromolecules 2004, 37, 5692–5702. (31) Meeker, D. L.; Mudigonda, D. S. K.; Osborn, J. M.; Loveday, D. C.; Ferraris, J. P. Macromolecules 1998, 31, 2943–2946. (32) McCulloch, I.; Zhang, W. M.; Heeney, M.; Bailey, C.; Giles, M.; Graham, D.; Shkunov, M.; Sparrowe, D.; Tierney, S. J. Mater. Chem. 2003, 13, 2436–2444. (33) McCulloch, I.; Bailey, C.; Genevicius, K.; Heeney, M.; Shkunov, M.; Sparrowe, D.; Tierney, S.; Zhang, W. M.; Baldwin, R.; Kreouzis, T.; Andreasen, J. W.; Breiby, D. W.; Nielsen, M. M. Philos. Trans. R. Soc. London, Ser. A 2006, 364, 2779–2787.

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Figure 1. Schematic illustration of the idealized concept of processing (i)

of telechelic oligomers (A) to create π-stacked domains (B) that can be further modified by photoinduced cross-linking (ii) to give cross-linked networks (C).

processing, we have recently taken interest in new π-conjugated materials that encompass the desired electroactive properties, processability, and stability and also provide an opportunity to incorporate the π-conjugated system covalently into a more complex macromolecular system. As illustrated in Figure 1A, we have been focusing on a series of discrete oligomers with an overall coil-rod-coil structure consisting of two alkyl spacers and a central π-conjugated oligomer segment. By adding terminal acrylate functionalities, we aim to exploit the selforganizing capability of the system induced by π-stacking between neighboring π-conjugated segments during processing (Figure 1B) and to introduce a second degree of connectivity by photoinduced cross-linking at the termini as depicted in a simplified manner in Figure 1C. Because of the extended connectivity, the soluble π-conjugated material is converted into an insoluble cross-linked structure. Moreover, if the cross-linking takes place without disrupting the intermolecular ordering and the electroactive properties drastically, then this approach can be used for photopatterning of optoelectronic devices with cooperative interactions between chromophores. This strategy has thus far resulted in the design of three new thiophene- and phenylene-based oligomeric systems, and we present here the synthesis and characterization of these oligomers as well as our work toward photopatterning and electrochromic display fabrication. Oligomer Synthesis and Characterization Synthesis of Model Oligomers. To establish the fundamental properties of the π-conjugated segments of the target compounds, we initially synthesized the two model compounds TPTTPT-Me2 and TPEEPT-Me2 (T for thiophene, P for phenylene, and E for EDOT), as depicted in Scheme 1.34 1,4-Dibromo-2,5-bis(dodecyloxy)benzene was converted to the 1-aryl-4-bromo-2,5-bis(dodecyloxy)benzenes 1 and 3 via Stille couplings with the appropriate 2-stannylated thiophenes using stoichiometric imbalance to ensure dominant monoreaction. Subsequent Negishi couplings with 2-chlorozinc-5-methylthiophene afforded the monomethylated bisthienylbenzenes (34) Wan, J. H.; Feng, J. C.; Wen, G. A.; Wang, H. Y.; Fan, Q. L.; Wei, W.; Huang, C. H.; Huang, W. Tetrahedron Lett. 2006, 47, 2829–2833. J. AM. CHEM. SOC.

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Scheme 1

2 and 4, which were then oxidatively homocoupled to give TPTTPT-Me2 and TPEEPT-Me2 in good yields. Structural Analysis. For X-ray diffraction studies, single crystals of TPTTPT-Me2 and TPEEPT-Me2 were grown, but unfortunately, all crystallization experiments afforded crystals that were not suitable for structural analysis. Instead, the model compounds PTTP and PEEP depicted in Figure 2 were designed to resemble the central conjugated moieties of TPTTPT-Me2 and TPEEPT-Me2, respectively. These model compounds were synthesized using a similar method (Supporting Information), and suitable single crystals were successfully obtained by slow evaporation of a dichloromethane/hexane mixture (PTTP) and a dichloromethane/acetonitrile mixture (PEEP). The crystal structures of PTTP and PEEP are displayed in Figures 3 and 4. Both molecules are located on inversion centers and have a completely planar central bithiophene moiety in the anti configuration. For PTTP, the outer phenyl rings are twisted out of the plane by 24.4(7)° relative to the inner thiophenes; favorable S-O interactions are evident from an intramolecular S-O distance of 2.747(1) Å, which is significantly shorter than the sum of the van der Waals radii (3.35 Å).35,36 Single crystals of PEEP adapt an even more coplanar structure with a dihedral angle between the inner thiophene and the outer phenyl ring of

only 8.0(2)°; S-O distances of 2.899(2) Å between the two EDOT rings and 2.627(2) Å between the alkoxy group and the thiophene again demonstrate highly favorable S-O interactions, which most likely contribute greatly to the coplanar arrangement. Furthermore, we note that the four dodecyl side chains in PEEP exhibit an all-trans configuration and are nearly coplanar to the aromatic core. Synthesis of Telechelic Oligomers. The synthesis of telechelic oligomers with identical conjugated backbones as the model compounds (Scheme 2) was achieved using a similar approach as described for the model oligomers above. Starting from the protected 6-(2-thienyl)-1-hexanol (5), which was easily synthesized from the corresponding bromide and 2-lithiothiophene, a Negishi coupling with either 1 or 3 afforded the monofunctionalized bisthienobenzenes 6 and 8. In this instance, homocoupling was carried out by lithiation and subsequent treatment with ferric acetylacetonate, since the previously applied ferric chloride conditions were incompatible with the hydroxyl protection group.37 Deprotection of 7 and 9 were easily achieved under mildly acidic conditions to give the functional oligomers TPTTPT-diol and TPEEPT-diol in excellent yields; further modification of the terminal hydroxy functionality with acryloyl chloride afforded TPTTPT-diacrylate and TPEEPT-diacrylate. Additionally, an all-thiophene oligomer was synthesized as described in Scheme 3. The dialkylated quaterthiophene 10 was dibrominated (11) and then successfully converted to the

Figure 2. Structure of model compounds PTTP and PEEP.

Figure 3. Molecular structure of a PTTP single crystal. 9736

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Figure 4. Molecular structure of a PEEP single crystal.

Discrete Photopatternable π-Conjugated Oligomers

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Scheme 2

Scheme 3

confirmed by elemental analysis and high-resolution mass spectrometry (see Supporting Information for experimental procedures and full characterization of all new compounds). Solution Properties

sexithiophene 12 in high yield by reaction with the THPprotected 6-(5-tributylstannyl-2-thienyl)-1-hexanol using modified Stille conditions inspired by the work of Fu and co-workers and Baldwin and co-workers.38,39 Deprotection to give T6-diol and subsequent conversion to T6-diacrylate were accomplished using the same protocol as described for the TPTTPT and TPEEPT systems in Scheme 2. All oligomers were characterized by 1H and 13C NMR spectroscopy and show narrow and well-defined signals as expected for monodisperse samples; identity and purity were (35) Turbiez, M.; Frere, P.; Roncali, J. J. Org. Chem. 2003, 68, 5357– 5360. (36) Turbiez, M.; Frere, P.; Allain, M.; Videlot, C.; Ackermann, J.; Roncali, J. Chem.-Eur. J. 2005, 11, 3742–3752. (37) Deprotection of compounds 6 and 8 and subsequent oxidative coupling with ferric chloride afforded TPTTPT-diol and TPEEPT-diol in similar overall yields. (38) Littke, A. F.; Schwarz, L.; Fu, G. C. J. Am. Chem. Soc. 2002, 124, 6343–6348. (39) Mee, S. P. H.; Lee, V.; Baldwin, J. E. Angew. Chem., Int. Ed. 2004, 43, 1132–1136.

Electrochemistry. Cyclic voltammetry (CV) of oligomers and polymers tends to give broad peaks, which can obstruct an accurate characterization of the redox-active system investigated. In differential pulse voltammetry (DPV), on the other hand, the faradaic current is extracted and the redox processes can consequently be analyzed more precisely. We have studied the redox properties of TPTTPT-Me2, TPEEPT-Me2, and T6-diol in solution by both CV and DPV, and the results are presented in Figure 5 and Table 1. The CV for TPTTPT-Me2 displays two well-resolved consecutive redox processes with half-wave potentials around 0.24 and 0.44 V (vs Fc/Fc+).40 Likewise, the voltammogram for TPEEPT-Me2 displays two fairly well-resolved consecutive redox processes with half-wave potentials around 0.15 and 0.50 V. The differential pulse voltammograms for TPTTPT-Me2 and TPEEPT-Me2 each reveal two well-defined and highly reversible anodic and cathodic processes with a high degree of symmetry; the half-wave potentials are now observed at slightly lower potentials than in the CV: 0.07 and 0.32 V for TPTTPTMe2 and at -0.01 and 0.40 V for TPEEPT-Me2. As expected, the first oxidation process occurs at a lower potential for TPEEPT-Me2 than for TPTTPT-Me2 because of the electrondonating ethylenedioxy substituents. The second oxidation process where the radical cation is converted to the dication occurs at a lower potential for TPTTPT-Me2 than for TPEEPTMe2. The same trend was observed for similar oligomeric systems by Roncali and co-workers and was ascribed to destabilization due to Coulombic repulsion between adjacent EDOT moieties in the dicationic state.41 The values for T6(40) In all of the CV experiments, a linear relationship between the oxidation current peak values and the square root of the scan rate was observed as expected for a diffusion-limited reaction at the electrode surface (see Supporting Information for voltammograms illustrating the scan rate dependence). (41) Turbiez, M.; Frere, P.; Allain, M.; Videlot, C.; Ackermann, J.; Roncali, J. Chem.-Eur. J. 2005, 11, 3742–3752. J. AM. CHEM. SOC.

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Figure 5. Cyclic voltammograms (A) and differential pulse voltammograms (B) of TPTTPT-Me2 (dashed lines), TPEEPT-Me2 (solid lines), and T6-diol (dotted lines) in dichloromethane. CV experiments performed with 0.1 mM oligomer and 0.1 M tetrabutylammonium perchlorate (scan rate 25 mV/s) and DPV with 1 mM oligomer and 0.1 M tetrabutylammonium hexafluorophosphate. Table 1. Summarized Electrochemical Properties for the Oligomers in Solutiona

TPTTPT-Me2b TPTTPT-Me2c TPEEPT-Me2b TPEEPT-Me2c T6-diolb T6-diolc

Epa,1

Epc,1

Epa,2

Epc,2

0.27 0.10 0.20 0.01 0.52 0.53

0.21 0.04 0.10 -0.03 0.46 0.49

0.48 0.35 0.57 0.42 0.64 0.66

0.39 0.30 0.42 0.38 0.58 0.62

a All potentials are reported in V vs ferrocene/ferrocenium. b CV data with 0.1 mM oligomer and 0.1 M tetrabutylammonium perchlorate in dichloromethane and scan rate 25 mV/s. c DPV data with 1 mM oligomer and 0.1 M tetrabutylammonium hexafluorophosphate in dichloromethane.

diol are in good agreement with earlier electrochemical studies of alkylated sexithiophenes under similar conditions.42,43 In comparison with the sexithiophene oligomer, the TPTTPT and TPEEPT systems have lower half-wave potentials as one would expect when taking into account their electron-rich 2,5-dialkoxy1,4-phenylene moieties. Optical Spectroscopy. All of the oligomers show a strong absorption band in the violet-to-blue region of the visible spectrum due to the π-π* transition. A broad and featureless absorption band is observed for the T6 (λmax 423-425 nm; εmax 45-49 000 M-1 cm-1) and the TPTTPT oligomers (λmax 430-432 nm; εmax 55 000-67 000 M-1 cm-1), whereas the TPEEPT system (λmax 444-445 nm; εmax 68 000-79 000 M-1 cm-1) shows vibronic resolution attributed to the fact that the EDOT units induce a more coplanar structure due to favorable S-O interactions between neighboring phenylene and EDOT rings as was illustrated in the crystal structure of PEEP (Figure 4). Chemical oxidation of TPTTPT-Me2 in dichloromethane with silver triflate affords a new transition with two peaks at 665 and 733 nm as well as a broad transition in the infrared (∼1630 nm) while the π-π* transition of the neutral state gradually disappears (Figure 6A). Investigation of the temperature dependence in this oxidized state (Figure 6B) reveals that the radical cation coexists with the π-dimer below room (42) Roncali, J.; Giffard, M.; Jubault, M.; Gorgues, A. J. Electroanal. Chem. 1993, 361, 185–191. (43) Nessakh, B.; Horowitz, G.; Garnier, F.; Deloffre, F.; Srivastava, P.; Yassar, A. J. Electroanal. Chem. 1995, 399, 97–103. 9738

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temperature;44–47 lowering of the temperature shifts the equilibrium from the radical cation (733, 1630 nm) toward the dimeric species (665, 1187 nm). Upon further oxidation with nitrosonium hexafluorophosphate, the absorption bands associated with the radical cation and the π-dimer species disappear and a new transition from the dication (1188, 1029 nm) is observed (Figure 6C). As also depicted in Figure 6 (inserted photographs), the neutral solution of TPTTPT-Me2 is yellow, the radical cation is blue, and the dication is quite transparent and color neutral since it mostly absorbs outside the visible region. Solution oxidation of TPEEPT-Me2 (Figure 7 and Table 2) gives results very similar to what was described for TPTTPTMe2 above. Additionally, the solution oxidation of T6-diol was examined (absorption spectra included in Supporting Information), and in agreement with previous optical studies of sexithiophenes,48,49 absorption bands were assigned to the neutral state (424 nm), the radical cation (806, 1557 nm), the π-dimer (702, 1124 nm), and the dication (893, 1003 nm). Compared to that of 3′,3′′′′-didodecylsexithiophene, which has unsubstituted R-positions, the absorption bands of T6-diol are red-shifted 20-50 meV (neutral and π-dimer species) to 60-80 meV (radical cation and dication species). While the neutral species and the dications of the oligomers show one HOMO-LUMO transition, the radical cations show two transitions, one corresponding to a HOMO-SOMO transition (low-energy transition) and one corresponding to a SOMO-LUMO transition (high-energy transition), as illustrated schematically in Figure 8. The electron-pairing energy stabilizes both the neutral and the dication ground states (but not the radical cation ground state), which explains why the absorption bands of the neutral species and the dication are observed at shorter wavelengths (44) Bauerle, P.; Segelbacher, U.; Maier, A.; Mehring, M. J. Am. Chem. Soc. 1993, 115, 10217–10223. (45) Hill, M. G.; Penneau, J. F.; Zinger, B.; Mann, K. R.; Miller, L. L. Chem. Mater. 1992, 4, 1106–1113. (46) Tabakovic, I.; Maki, T.; Miller, L. L.; Yu, Y. A. Chem. Commun. 1996, 1911–1912. (47) Hill, M. G.; Mann, K. R.; Miller, L. L.; Penneau, J. F. J. Am. Chem. Soc. 1992, 114, 2728–2730. (48) Bauerle, P.; Segelbacher, U.; Gaudl, K. U.; Huttenlocher, D.; Mehring, M. Angew. Chem., Int. Ed. 1993, 32, 76–78. (49) Fichou, D.; Horowitz, G.; Xu, B.; Garnier, F. Synth. Met. 1990, 39, 243–259.

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Figure 6. Solution oxidation of TPTTPT-Me2 in dichloromethane. (A) Conversion from neutral to radical cation and π-dimer species, (B) temperature

dependence of the monomer/dimer equilibrium, (C) conversion from radical cation to dication, and photographs of the observed color changes.

than the high- and low-energy transitions of the radical cation, respectively. The two transitions from the π-dimer are blueshifted (0.17-0.23 eV for the high-energy transition and 0.19-0.30 eV for the low-energy transition) compared to the monomeric radical cation; this is indicative of a face-to-face interaction and is normally referred to as a Davydov blue-shift.44 The absorption bands from the radical cation and the π-dimer overlap significantly for all three oligomers, which makes it complicated to determine the dimer formation enthalpy. Qualitatively, the dimer formation enthalpy is found to be largest for the TPTTPT system and smallest for the TPEEPT system. This is most likely governed by the degree of charge delocalization; the radical cation of TPEEPT has a more localized charge distribution than TPTTPT and T6 and will therefore, because of charge repulsion, be less prone to form a π-dimer. For the dications, we observe vibronic fine structure in all three cases, a phenomenon caused by a more quinoidal structure and hence increased planarity in the oxidized systems. We furthermore note that these vibronic splittings fall in the range ∆E ) 0.15-0.17 eV, which is in good agreement with previous studies of other oligomeric thiophene systems.44,45,50 Electron Paramagnetic Resonance Spectroscopy. Electron paramagnetic resonance (EPR) spectroscopy of TPTTPT-Me2 and TPEEPT-Me2 in dichloromethane confirmed the presence of the paramagnetic radical cation upon oxidation (the π-dimer is EPR silent) and the subsequent conversion to the EPR silent dication upon further oxidation (Figures 9 and 10). The radical cation of TPTTPT-Me2 has an EPR signal centered at a g value

of 2.012 with a peak-to-peak width of 5.3 G, while the signal from radical cation of TPEEPT-Me2 is centered at g ) 2.008 with ∆Hpp ) 4.2 G; these EPR characteristics compare well to earlier studies of oligothiophenes by Tour and co-workers.50 No hyperfine coupling was observed for either of the two systems, a finding that could be attributed to the delocalization of the radical cation wave function, which results in sampling a large number of different isotropic interactions with neighboring hydrogens. While hyperfine coupling has been observed for short oligothiophenes,46,47 longer π-conjugated oligomers generally give rise to broad and featureless EPR signals.51,52 The slightly broader line width observed for the TPTTPT system than that for the TPEEPT system is most likely due to the larger number of hydrogens attached directly to the aromatic core of TPTTPT-Me2 (12 vs eight hydrogen atoms). Incomplete conversion from the radical cation to the dication explains the small EPR signals observed for the dication preparation of both TPTTPT-Me2 and TPEEPT-Me2 (Figure 10, dotted lines). Solid-State Properties Electrochemistry. To investigate the redox properties of the oligomers in the solid state, we focused on thin films of the (50) Guay, J.; Kasai, P.; Diaz, A.; Wu, R. L.; Tour, J. M.; Dao, L. H. Chem. Mater. 1992, 4, 1097–1105. (51) Janssen, R. A. J.; Moses, D.; Sariciftci, N. S. J. Chem. Phys. 1994, 101, 9519–9527. (52) Kanemoto, K.; Kato, T.; Aso, Y.; Otsubo, T. Phys. ReV. B 2003, 68, 092302. J. AM. CHEM. SOC.

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Figure 7. Solution oxidation of TPEEPT-Me2 in dichloromethane. (A) Gradual conversion from neutral to radical cation and π-dimer species, (B) temperature

dependence of the monomer/dimer equilibrium, (C) conversion from radical cation to dication, and photographs of the observed color changes. Table 2. Absorption Maxima (nm) for the Neutral (M), Radical Cation (M+ · ), π-Dimer (M22+), and Dication (M2+) of TPTTPT-Me2, TPEEPT-Me2, and T6-Diol in Dichloromethane Solution (10-5 M).

M M•+ M22+ M2+

TPTTPT-Me2

TPEEPT-Me2

T6-diol

430 733, 1630 665, 1187 1029, 1188

445 705, 1389 630, 1198 1007, 1167

424 806, 1557 702, 1124 893, 1003

diacrylate oligomers; upon UV irradiation of these films, the diacrylates polymerize to form cross-linked networks, which renders them insoluble in all common organic solvents and hence suitable for solid-state electrochemistry. Redox properties of the cross-linked films in acetonitrilesupported electrolyte are depicted in Figure 10, and the results are summarized in Table 3. For TPEEPT-diacrylate, the two consecutive redox processes are well-resolved, whereas two poorly resolved peaks (TPTTPT-diacrylate) or one broad peak with shoulder features (T6-diacrylate) are observed for the other two systems. Some electrochemical irreversibility is expressed through the lower current passed during the cathodic sweep as compared to the current passed during the anodic sweep. Hysteresis effects are observed for all three systems, and potential differences as large as 450 mV between the oxidation and the reduction peaks are seen for T6-diacrylate. From the onset of oxidation, it is estimated that T6-diacrylate has a HOMO value of 5.4 eV; changing the two 3-alkylthiophenes into two 2,5-dialkoxy-1,4-phenylenes does not affect the HOMO value as it is found to be 5.4-5.5 eV for TPTTPT9740

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diacrylate also. Subsequent conversion of the two inner thiophenes into the more electron-rich EDOTs raises the HOMO value to approximately 5.2 eV as observed for TPEEPTdiacrylate. Numerous studies have illustrated that dioxythiophenecontaining systems have a significantly increased stability of the oxidized states as compared to unsubstituted thiophene and alkylthiophene systems.53–57 This observation is also made in this study when comparing the redox processes of TPTTPTdiacrylate, TPEEPT-diacrylate, and T6-diacrylate and is particularly pronounced when investigating the solid-state electrochemistry in a dichloromethane-supported electrolyte. While TPTTPT-diacrylate and T6-diacrylate films in dichloromethane show limited current responses upon electrochemical oxidation and continuously lose electroactivity, incorporation of EDOT moieties greatly enhances the redox stability. This is illustrated for TPEEPT-diacrylate in Figure 11; both the CV and the DPV experiments reveal two well-separated oxidation peaks followed by two reduction peaks offering a high degree of symmetry and thus a high degree of chemical reversibility. The first half-wave potential is observed at 0.05 V in both CV (53) Jonas, F.; Heywang, G. Electrochim. Acta 1994, 39, 1345–1347. (54) Yamato, H.; Ohwa, M.; Wernet, W. J. Electroanal. Chem. 1995, 397, 163–170. (55) Pei, Q.; Zuccarello, G.; Ahlskog, M.; Inganas, O. Polymer 1994, 35, 1347–1351. (56) Dietrich, M.; Heinze, J.; Heywang, G.; Jonas, F. J. Electroanal. Chem. 1994, 369, 87–92. (57) Heywang, G.; Jonas, F. AdV. Mater. 1992, 4, 116–118.

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Figure 8. (A) Frontier molecular orbitals and dipole-allowed transitions for a neutral species (M), a radical cation (M•+), and a dication (M2+). (B) Frontier orbitals and allowed transitions for the π-dimer based on linear combination of molecular orbitals for the radical cation.

Figure 9. EPR spectra of the neutral (solid line), radical cation (dashed line), and dication (dotted line) of (A) TPTTPT-Me2 and (B) TPEEPT-Me2 in dichloromethane (1 mM).

Figure 10. Cyclic voltammograms (A) and differential pulse voltammograms (B) of cross-linked films of TPTTPT-diacrylate (dashed lines), TPEEPTdiacrylate (solid lines), and T6-diacrylate (dotted lines) on ITO-coated glass in 0.1 M tetrabutylammonium perchlorate in acetonitrile (CV scan rate 150 mV/s).

and DPV (Table 3), while the second half-wave potential is identified at 0.64 V (CV) and 0.56 V (DPV).40 These potentials are significantly lower than the values obtained in acetonitrilesupported electrolyte but compare well with the solution data for TPEEPT-Me2 also obtained in dichloromethane. From the onset of oxidation, the HOMO energy level for TPEEPTdiacrylate is estimated to be approximately 5.0 eV under these conditions. The electrochemical results presented here on the three oligomer systems in the solid state clearly show that crosslinked films of TPEEPT-diacrylate are the most electro-

chemically stable materials of the three. Films of TPTTPTdiacrylate and T6-diacrylate show poor electrochemical stability in both acetonitrile and dichloromethane. Films of TPEEPT-diacrylate, on the other hand, switch nicely, especially in a dichloromethane-supported electrolyte, between a yellow neutral state, a blue radical cation state, and a more transparent blue/purple dication state. Thus, compared to that of TPTTPT and T6, the TPEEPT chromophore proves to be a far more promising candidate for electrochromic applications, and the diacrylate functionality offers a convenient means of controlling solubility. J. AM. CHEM. SOC.

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Table 3. Summarized Electrochemical Properties for the Diacrylate Oligomers in the Solid Statea

b

TPTTPT TPTTPTc TPEEPTb TPEEPTc T6b T6c TPEEPTd TPEEPTe

Eonset

Epa,1

Epc,1

Epa,2

Epc,2

0.41 0.33 0.20 0.06 0.33 0.35 -0.06 -0.13

0.72 0.68 0.39 0.27 0.70 0.58 0.07 0.10

0.52 0.62 0.37 0.20 0.25 0.53 0.03 0.00

1.05 0.98 0.99 0.73 0.91

0.93 0.94 0.89 0.71 0.46

0.66 0.61

0.61 0.50

a All potentials are reported in V vs ferrocene/ferrocenium. All experiments were performed with cross-linked films on ITO-coated glass with 0.1 M tetrabutylammonium perchlorate as electrolyte. b CV data in acetonitrile and scan rate 150 mV/s. c DPV data in acetonitrile. d CV data in dichloromethane and scan rate 25 mV/s. e DPV data in dichloromethane.

Optical Spectroscopy. Thin film absorption spectra (Figure 12, Table 4) indicate the expected more coplanar structure in the solid state compared to solution for all three systems, although indications of a less ordered packing are observed for TPEEPT-diacrylate. Figure 12 additionally pictures the changes in the optical properties when cross-linking the spray-cast films. The cross-linking process is associated with a small blue-shift (7-11 nm) for TPTTPT-diacrylate and T6-diacrylate, which indicates that these two systems rearrange to slightly less coplanar conformations upon the photopolymerization. TPEEPTdiacrylate, on the other hand, adapts a more coplanar structure overall during the cross-linking process as judged from the increased intensity of the low-energy peaks and decreased intensity of the high-energy peaks. For all three systems, loss of order during the UV irradiation is indicated by the loss of vibronic coupling in the absorption spectra. It is conceivable that the cross-linking process is associated with molecular rearrangements that would lead to an overall loss of order in the film.32 The optical band gaps, as determined from the lowenergy band-edges, fall in the range 2.3-2.4 eV. Spectroelectrochemistry. To further demonstrate the high stability of the TPEEPT system in its oxidized states, spectroelectrochemistry was carried out for a spray-cast and subsequently cross-linked film of TPEEPT-diacrylate as depicted in Figure 13. At -0.24 V (vs Fc/Fc+), the film is neutral and therefore displays only the π-π* transition at 452 nm. As the potential is increased stepwise to 0.34 V, the absorption band from the neutral film disappears gradually, while two broad bands ascribed to the radical cation (monomeric and/or dimeric) emerge at 603 and 1180 nm; the latter blue-shifts gradually to 1140 nm as it increases in intensity (Figure 13A). Further increment of the potential to 0.86 V is accompanied by the gradual appearance of a transition from the dication (1000 nm) at the expense of the absorption bands from the radical cation as illustrated in Figure 13B. Visualization of the optical changes is aided by observation of the difference spectra obtained by subtraction of the first spectrum in a series from the following spectra as illustrated in Figure 13C,D. Especially for the conversion from radical cation to dication, the regular absorption spectra are complicated by the overlap of the radical cation and dication absorption bands, but this is overcome by examination of the difference spectra instead. This deconvolution reveals the gradual disappearance of the two bands from the radical cation and the corresponding appearance of the dication absorption band. It is furthermore revealed by this approach that a low-intensity absorption band around 685 nm is associated with the dication absorption band at 1000 nm. The inserted 9742

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photographs show the distinct color changes from a bright yellow neutral film to a clear blue (radical cation), and subsequently on to an almost transparent dicationic state with a residual bluish component from the radical cation and a slight purple color from the 685-nm band. Comparison with the solution oxidation data (Figure 7) reveals that the positions of the various absorption bands are more or less unchanged during the transition from solution to the solid state. However, the bands in the solid state are broadened and lack the resolution observed in solution, most likely due to the rigidification of the films taking place during cross-linking, which locks the conformation and hinders any substantial reorganization in the film during the oxidation processes. Photopatterning and Switching Studies. After having established the chemical stability of the TPEEPT system in its various oxidized states, the high degree of redox reversibility, and excellent multicolored electrochromic behavior, the next step was to employ the diacrylate functionality of TPEEPTdiacrylate for photopatterning purposes. Thin films of TPEEPTdiacrylate cross-link successfully within a few hours with no sign of degradation of the TPEEPT chromophores. Figure 14 shows how the absorption increases and the emission decreases when the films are thermally annealed and subsequently photopolymerized, a finding that indicates that the degree of structural order is increasing during the process.58 As illustrated in Figure 15, this strategy (in combination with simple mask fabrication) can be applied in a straightforward photopatterning process to create defect-free patterns with at least submillimeter resolution. Photopatterned films of TPEEPTdiacrylate show the same optical, electrochemical, and spectroelectrochemical characteristics as already discussed for homogeneous films of TPEEPT-diacrylate; the capability of manifesting three distinct colors when going from neutral to radical cation and to dication is also conserved as obvious from Figure 15B-D. To further probe the electrochromic properties of the crosslinked TPEEPT-diacrylate material, the switching characteristics for several films were investigated by monitoring the transmittance at the wavelength of maximum contrast during repeated redox stepping experiments (Table 5). Figure 16 shows the chronoabsorptometry at a wavelength of 600 nm for the electrochromic switching between the neutral state (yellow, λmax 452 nm) and the radical cation (blue, λmax 603 nm) with 30-s delays between each redox switch. For a relatively thick film (225 nm, Figure 16A), the percent transmittance is 89.2% in the bleached state (yellow) and 26.7% in the colored state (blue), which gives a contrast value of 62.5% (∆%T). For a thinner film (85 nm, Figure 16B), the percent transmittances are 99.3% (bleached) and 61.5% (colored), resulting in a contrast ratio of 37.8%; after 400 switches (7 h) of the thinner film, the contrast ratio decayed to approximately 30%. A good correlation was observed between film thickness and contrast ratio as obvious from the data in Table 5 (see Supporting Information for the complete set of switching data). Response times for achieving the blue state (600 nm coloration, tc) and the yellow state (600 nm bleaching, tb) were determined as the time it took to reach 95% of the ultimate change in %T.59 As expected, the switching times were highly dependent on film thickness: for the thinner films, response times of 2.3-2.4 s for coloration and ap(58) Kim, Y.; Cook, S.; Tuladhar, S. M.; Choulis, S. A.; Nelson, J.; Durrant, J. R.; Bradley, D. D. C.; Giles, M.; McCulloch, I.; Ha, C. S.; Ree, M. Nat. Mater. 2006, 5, 197–203.

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Figure 11. Cyclic voltammograms (A) and differential pulse voltammograms (B) of cross-linked films of TPEEPT-diacrylate on ITO-coated glass in 0.1

M tetrabutylammonium perchlorate in dichloromethane (CV scan rate 25 mV/s).

switching time observed for the bleaching process, which is approximately four times slower than the coloration process. The cross-linking process is likely to cause a tighter organization with more closely packed chains. Therefore, the coloration process, which takes the oligomer from its neutral to its oxidized form, should take place easily since it is associated with a planarization of the oligomer backbone as the quinoidal radical cation forms. On the other hand, the bleaching process is associated with the conversion from the quinoidal oxidized species to the slightly less planar neutral form and could therefore be hindered by the tight locked packing in the crosslinked network. Further indication of the tight packing is also seen with the significantly increased switching times with increased film thickness. Conclusions Figure 12. Normalized absorption spectra of films of TPTTPT-diacrylate (top), TPEEPT-diacrylate (middle), and T6-diacrylate (bottom) spraycast from toluene/dichloromethane (9:1) before (dashed lines) and after (solid lines) cross-linking. Table 4. Summarized Optical Properties for the Oligomers λmax (nm)a

λmax (nm)b

λmax (nm)c

Eonset (eV)c

432 445 425

454, 480 420, 447, 485 444

443 452, 482 437

2.4 2.4 2.3

TPTTPT-diacrylate TPEEPT-diacrylate T6-diacrylate c

a In THF solution. Cross-linked films.

b

Films spray-cast from toluene/dichloromethane.

proximately 10 s for bleaching were observed, while the thicker films showed coloration times of 3.6 s (115 nm film) and 4.6 s (225 nm film) and bleaching times of 14.5 s (115 nm) and 21.1 s (225 nm). Apart from the slow bleaching process, these switching and contrast characteristics are comparable to other EDOT-containing electrochromic materials described in the literature,60 but it should be noted that other electrochromic π-conjugated polymers with much faster switching times (100-300 ms) and enhanced lifetimes have been reported.25,61 The instability observed during prolonged switching is primarily caused by an incomplete return to the neutral state (Figure 16B) and seems to be connected with the relatively long (59) Gaupp, C. L.; Welsh, D. M.; Rauh, R. D.; Reynolds, J. R. Chem. Mater. 2002, 14, 3964–3970.

We presented two novel chromophores (TPTTPT and TPEEPT), which were synthesized both as methyl-terminated model compounds and telechelic analogues together with two novel telechelic sexithiophene derivatives (T6-diol and T6diacrylate). Structural characterization based on single crystal structures of PTTP and PEEP indicates that a central biEDOT unit (due to attractive intramolecular sulfur-oxygen interactions) causes a higher degree of molecular planarity than a bithiophene unit. Comparison with the crystal structure of 3,3′′′-didocylquaterthiophene62 suggests a close resemblance between the PEEP motif and the dialkylated quaterthiophene, while PTTP is somewhat less planar. The overall structural rigidifying effect of the EDOTs is confirmed by the vibronic resolution observed in the optical absorption spectra for TPEEPT, but not for TPTTPT or T6. In dichloromethane, we found the oxidized species of all three oligomeric systems to be quite stable, which allowed for a thorough characterization of the redox properties and the different oxidized species involved. The all-thiophene T6 system has the highest first oxidation potential; introduction of the more (60) Kumar, A.; Welsh, D. M.; Morvant, M. C.; Piroux, F.; Abboud, K. A.; Reynolds, J. R. Chem. Mater. 1998, 10, 896–902. (61) Lu, W.; Fadeev, A. G.; Qi, B. H.; Smela, E.; Mattes, B. R.; Ding, J.; Spinks, G. M.; Mazurkiewicz, J.; Zhou, D. Z.; Wallace, G. G.; MacFarlane, D. R.; Forsyth, S. A.; Forsyth, M. Science 2002, 297, 983–987. (62) Azumi, R.; Gotz, G.; Debaerdemaeker, T.; Bauerle, P. Chem.-Eur. J. 2000, 6, 735–744. J. AM. CHEM. SOC.

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Figure 13. Spectroelectrochemistry for a cross-linked film of TPTTPT-diacrylate on ITO-coated glass in 0.1 M tetrabutylammonium perchlorate in

dichloromethane. (A) Gradual conversion from neutral to radical cation species (potential stepped from -0.24 to 0.34 V vs Fc/Fc+), (B) conversion from radical cation to dication (potential stepped from 0.34 to 0.86 V vs Fc/Fc+), (C) difference absorption spectra showing the optical changes during the conversion from neutral to radical cation, and (D) difference absorption spectra for the conversion from radical cation to dication.

Figure 14. UV-vis (A) and fluorescence (B) spectra of a thin film of TPEEPT-diacrylate. Film was spray-cast and then annealed (60 °C in vacuum

overnight) before being cross-linked (UV-curing for 10 min) and then rinsed with tetrahydrofuran.

electron-rich dialkoxybenzene unit lowers the oxidation potential (TPTTPT), and a further decrease is seen when introducing the EDOT unit. Although TPEEPT thus has the most stable radical cation, the next one-electron oxidation occurs more easily for T6 (∆E ) 120-130 mV) and for TPTTPT (∆E ) 200-250 mV) than for TPEEPT (∆E ) 350-410 mV) because of charge repulsion in the dication of TPEEPT due to a lower degree of charge delocalization along the backbone. These electrochemical 9744

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properties are reflected very well by the optical properties of the oxidized species. The red-shift associated with the first oxidation (from neutral to radical cation) is small for TPEEPT (1.03 eV), somewhat larger for TPTTPT (1.19 eV), and even larger for T6 (1.38 eV). For the second oxidation (from radical cation to dication), on the other hand, the trend is opposite with the red-shift being only 0.30 eV for T6, 0.65 eV for TPTTPT, and 0.70 eV for TPEEPT. Each of the radical cations is in

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Figure 15. Photopatterning process for a spray-cast film of TPEEPTdiacrylate (A). After (i) applying a mask, (ii) irradiating with UV light, and (iii) washing with tetrahydrofuran, we found that the cross-linked and patterned film appears as illustrated in (B). Subsequent electrochemical oxidation affords the blue radical cation (C) and the pale blue/transparent dication (D). Table 5. Summarized Switching Properties for Four Cross-Linked TPEEPT-Diacrylate Films of Various Thicknesses film thickness (nm)

∆%T

tc (s)

tb (s)

80 85 115 225

34.4 37.8 45.4 62.5

2.4 2.3 3.6 4.6

10.4 9.6 14.5 21.1

a All experiments were performed with cross-linked films on ITO-coated glass with 0.1 M tetrabutylammonium perchlorate as electrolyte. Switching was monitored as the percent transmittance (%T) at 600 nm with a 30-s delay for each potential (-0.65 and 0.25 V), and switching times were determined as the time it took to reach 95% of the ultimate change in %T.

equilibrium with its corresponding π-dimer in solution; again, we see evidence for the fact that the EDOTs tend to favor a more localized charge distribution since TPEEPT is less prone to dimerization than TPTTPT and T6. The herein reported oligomers are all highly soluble in many common organic solvents and can thus easily be spin- or spraycast onto various substrates to form thin homogeneous films. Furthermore, by the attachment of acrylate functionalities to the oligomers, we were able to convert the solution-processed films into insoluble cross-linked networks by UV curing. Thus,

we can take advantage of the very convenient solution processibility as well as subsequent solid-state processing, which “locks” the film into an insoluble solid state. In general, we found the oligomers to behave very similarly in the solid state to what was observed in solution. The cross-linked film of TPEEPT-diacrylate has the lowest first oxidation potential, but opposite to what was observed in solution, T6 (E1/2 ) 0.47 V) is now more easily oxidized to the radical cation than TPTTPT (E1/2 ) 0.62 V), which is probably caused by a more coplanar organization for T6 in the solid state than for TPTTPT as also indicated in the single crystal structures. Again, we observe the next one-electron oxidation to the dication to occur more easily for T6 (∆E ) 210 mV) and for TPTTPT (∆E ) 310-370 mV) than for TPEEPT (∆E ) 460-480 mV). In particular, we discovered that a cross-linked film of TPEEPT-diacrylate is quite stable in its different oxidized states and thus undergoes two consecutive and very reversible redox processes even under ambient conditions. Further characterization by spectroelectrochemistry reveals two distinct color changes when going from the bright yellow neutral material to the clear blue radical cation and subsequently to the dication, which is transparent with a small residual gray color. The promising electrochromic properties of this material are further verified by our initial switching studies showing good contrast ratios and reasonable coloration and bleaching times during at least 400 cycles. To utilize the cross-linkable acrylate functionality to its fullest, we have (as a proof of concept) shown how a TPEEPT-diacrylate film successfully can be subjected to a simple photopatterning process. To summarize, we developed a series of telechelic oligomers with various π-conjugated cores. From our studies, it is obvious that 3,4-dioxythiophene moieties (here in the form of EDOT units) offer an enhanced stability of the oxidized states, which makes these oligomers superior for electrochromic devices as compared to the thiophene- and phenylene-based systems. Further synthetic efforts including modifications of the 3,4dioxythiophene (incorporation of ProDOT, dialkyl-ProDOT, etc.), the relative positions of the 3,4-dioxythiophene units in the oligomer, and variations in the length of the oligomer and the attractive intramolecular forces determining the molecular planarity could help identify new electrochromic materials with altered optical properties. In addition to the presented electrochromic properties of TPTTPT, TPEEPT, and T6, these oligomeric systems are also promising candidates for new transistor materials. Therefore, we are currently investigating charge-carrier mobilities of these

Figure 16. Electrochromic switching of cross-linked TPEEPT-diacrylate films (0.1 M tetrabutylammonium perchlorate in dichloromethane) monitored as

the percent transmittance (%T) at 600 nm with a 30-s delay for each potential (-0.65 and 0.25 V). Ten switching cycles for a 225-nm film (A) and first 10 and last 50 switching cycles for an 85-nm film (B). J. AM. CHEM. SOC.

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oligomers in our laboratories, while also pursuing a deeper understanding of the morphologies of these materials in their solid state. Acknowledgment. We appreciate funding of this work by the AFOSR (FA9550-06-1-0192) and via a grant to EIC Laboratories (FA9550-05-C-0147). K.A.A. acknowledges the National Science Foundation and the University of Florida for funding the purchase of the X-ray equipment. We thank Geeta Kheter Paul for the supply of compound 10. Supporting Information Available: Experimental procedures including full characterization for all new compounds. Cyclic

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voltammograms showing the scan rate dependence for TPTTPT-Me2, TPEEPT-Me2, and T6-diolin solution as well as for TPEEPT-diacrylate in the solid state. UV-vis spectra displaying the solution oxidation of T6-diol. Chronoabsorptometry showing the electrochromic switching of cross-linked TPEEPT-diacrylate films. Listings of crystal data and structure refinement parameters, atomic coordinates, bond lengths and angles, anisotropic displacement parameters, hydrogen coordinates, and isotropic displacement parameters for PTTP and PEEP (PDF and CIF). This material is available free of charge via the Internet at http://pubs.acs.org. JA7112273